Integrated networks transmit voice, video, and data to subscribers via network signal carriers in the form of coaxial cable or optical fiber. An example of an integrated network is a passive optical network (PON), which can deliver voice, video, and data, often referred to as “triple play services.” A PON delivers voice, video and data among multiple network nodes, often referred to as optical network terminations (ONTs), using a common optical fiber link. Passive optical splitters and combiners enable multiple ONTs to share the optical fiber link. Each ONT terminates the optical fiber link for a residential or business subscriber, and is sometimes referred to as a subscriber premises node. An ONT is connected to one or more subscriber devices, such as televisions, set-top boxes, telephones, computers, or network appliances, which ultimately receive the voice, video and other data delivered via the PON.
Generally, a PON includes a PON interface, sometimes referred to as an optical line terminator (OLT), having multiple, independent PON interface modules that serve multiple optical fiber links. A PON interface module provides an interface for transmission and reception of data packets over a particular optical fiber link that serves a group of ONTs. A PON is inherently a downstream-multicast medium. Each packet transmitted on an optical fiber link can be received by every ONT served by that link. ONTs identify selected packets or frames on the fiber link based on addressing information included within the packets or frames.
In a fiber to the premises (FTTP) application, each ONT terminates the optical fiber link for a residential or business subscriber, and is sometimes referred to as a subscriber premises node. The OLT is typically located in a telecommunication company central office (CO), while the ONT is typically mounted on or within a residence or business. For an exemplary FTTP application, the triple play services are transmitted over the PON on three wavelengths of light, e.g., 1550 nanometer (nm) for downstream analog video, 1490 nm for downstream digital voice and data, and 1310 nm for upstream digital voice and data.
On the OLT side of the fiber, a 1490 nm transmitter and a 1310 nm receiver are contained in an optical diplexer. As an example, the optical diplexer may contain a 1490 nm laser, a 1310 nm positive-intrinsic-negative (PIN) photodiode along with a Trans-Impedance Amplifier (TIA), and Coarse Wavelength Division Multiplexer (CWDM) optics to separate the 1490 and 1310 nm wavelengths of light. The 1550 nm light carrying analog video from a headend cable television (CATV) laser is coupled to the PON via external CWDM optics.
At the ONT, a single optical component called a triplexer separates the three wavelengths received via the optical fiber into three ports. A laser driver controls a 1310 nm laser to transmit upstream data from the ONT onto the optical fiber. A PIN photodiode plus a TIA operate to receive the 1490 nm downstream data from the optical fiber, and an analog photodiode receives the 1550 nm video signal from the optical fiber. CWDM optics also are provided in the ONT to separate the light for the three different ports, i.e., digital transmit, digital receive and analog video.
A triplexer module generally includes the triplexer, the laser driver to interface to the laser, the limit amplifier to amplify the output of the low level digital receiver, and the video circuit. More particularly, the video circuit in the triplexer module includes the analog photodiode, an analog photodiode matching network, an automatic gain controller, a radio frequency (RF) amplifier and an RF output connector to a television. The limit amplifier in the receiver transmits digital serial receive data to the ONT media access control (MAC) circuitry, which directs data to provide data and voice applications to a subscriber, e.g., for Ethernet and telephone services. The laser driver receives digital serial transmit data from the ONT MAC for transmission of data via the optical fiber.
Most FTTP installations require voice and data, but only approximately half of these installations require analog video for CATV service. Consequently, a less expensive diplexer can be used on the ONT side, instead of a triplexer module, if no analog video is required. A diplexer is generally constructed in a manner similar to a triplexer, except that it does not include an analog photodiode and associated optics for analog video applications. The diplexer also eliminates the analog circuitry associated with the CATV output of the FTTP system. Triplexer and diplexer modules typically are fixed modules that are soldered to an ONT circuit card, and are not ordinarily footprint-compatible. Consequently, vendors often carry in inventory two different versions of the ONT circuit card, one with a triplexer for analog video and the other with a diplexer if analog video is not required.